Sowing Intercropped Rapeseed
There is increasing talk of " associated rapeseed", particularly in practices of conservation agriculture. The technique consists of associating a cover crop with rapeseed from the time of crop establishment. This cover remains in place in autumn and is destroyed during winter. Establishing a cover crop has several benefits related to issues concerning nitrogen, pests, and weeds.
Principle
Associating rapeseed with other crops helps maintain or even increase rapeseed yields while reducing the need for nitrogen fertilization and weed control. This rapeseed-legume association is suitable for situations with low nitrogen availability, especially in western and central France. The benefit is more limited when nitrogen availability at sowing is high (organic fertilizer inputs, protein crop predecessors, high post-harvest residues, etc.).
Objectives
Nitrogen restitution for rapeseed
The cover destroyed during winter (especially if it contains legumes) will return soil organic matter and nitrogen to the soil, which rapeseed can benefit from in spring. A well-established cover can restitute on average between 20 to 30 kg of nitrogen available for rapeseed.
In spring, nitrogen release varies depending on the legume species according to their growth habit and Carbon/Nitrogen ratio. Faba bean with its upright habit has less contact with the soil (40% of its biomass will mineralize quickly, stems remain in place), whereas a vetch or fenugreek with their sprawling habit have 100% of their aerial biomass in contact with the soil, thus releasing nitrogen more rapidly.
This highlights the interest in associating several species to improve the timing of nitrogen release for the intercrop period and the following crop.
Nitrogen valorization
The association allows a valorization of mineral nitrogen of about 10 to 20 units linked to better soil porosity. Establishing an associated cover with rapeseed further promotes nitrogen uptake in autumn and helps reduce nitrogen leaching risks.
Competition against weeds
A well-established associated rapeseed accompanied by legumes helps reduce weed pressure (geranium, cleavers, chamomiles).
The presence of companion plants blocks light from reaching the soil, resulting in a reduction in weed emergence. This aspect is interesting for reducing herbicide use.
Trials conducted by Terres Inovia on 15 farms from 2013 to 2016 comparing associated rapeseed and sole rapeseed showed a correlation between the aerial fresh weight of rapeseed and weed presence in the fields. It was observed that beyond 1,500 g/m2, the number of weeds per m2 decreases significantly with associated rapeseed treatments.
Structuring effect
The associated cover can improve soil structure, which may promote rapeseed root development. 90% of legume roots are located in the top 10 centimeters, which induces improved soil porosity and better rooting of rapeseed (the taproot of associated rapeseed is 3-4 cm deeper compared to sole rapeseed).
Disrupts autumn pest attacks
Rapeseed associated with mixtures based on vetch or faba bean seem less attractive to the cabbage stem weevil and the rape flea beetle. The presence of companion plants with rapeseed causes disruption among pest insects and reduces the risk of damage to the crop:
- Visual disruption: the insect recognizes the color and shape of the host plant. This is accentuated by a strong contrast between bare soil and the developing host plant. The presence of a companion plant reduces this contrast.
- Olfactory disruption: insects are attracted by volatile compounds (constituting the specific odor of the host plant). The companion plant masks or disturbs the odor and hinders insect orientation.
- Appropriate/inappropriate landing theory: the insect lands on a plant and stays if it is its host plant or takes off again either to leave the plot or to find the correct one;
- Resistance by association: trap or decoy plant – the host plant benefits from the protection of its companion plant.
Trials conducted by Greenotec in 2019 comparing damage from flea beetles on rapeseed associated and sole rapeseed showed a clear advantage for associated rapeseed. On 4 out of 5 trial plots, a reduction in bite damage ranging from -20% to -36% was observed for associated rapeseed compared to sole rapeseed.
Increase economic profitability
Trials conducted by Terres Inovia between 2013 and 2016 showed gross margins multiplied by 1.35 to 2 compared to sole rapeseed, 13% to 18% less nitrogen, a 25 to 35% reduction in input costs, 45 to 55% reduction in phytosanitary costs, and IFT halved.
Trials conducted in 2018 by Anthony Frison showed interesting profitability results for treatments with companion plants for all types of tillage (+5% gross margin for associated rapeseed compared to sole rapeseed in CT, +2.5% in strip till and +4% in no-till), except for plowing (-1%).
Choice of cover
The choice of associated cover must be considered according to 4 main criteria:
- A cover based on legumes to capture atmospheric nitrogen.
- A cover that does not compete with rapeseed.
- A cover resistant to rapeseed herbicides.
- A frost-sensitive cover in winter.
According to equipment
According to plant relationships
Play on the complementarity of cover crop benefits: complementary rooting to rapeseed to improve soil exploration, good nitrogen storage capacity in autumn then release in spring, soil cover to compete with weeds and seek sufficient vegetation height to disrupt insects.
Generally, crucifers like turnip rape should be avoided and the easiest associated cover to succeed is faba bean.
Decision tools
To help you in your choice, INRAe has developed the decision support tool CAPS (Rapeseed Associated with Service Plants), which you can download via this link.
Associations with legumes
Legumes are later than rapeseed. They need at least 500°C days to start dynamic growth. Rapeseed emerges earlier and has a shorter dynamic growth phase, giving it systematically at least one week advantage over legumes.
This aspect limits competition risks on rapeseed during the first phase of the cycle and makes the association functional. The earliness index is an important parameter to consider for natural destruction in winter (destruction by low temperatures is effective only if legumes reach the bud stage).
| Cover | Advantages | Disadvantages | Frost sensitivity | Plant habit | Appeal to | Sclerotinia risk | Seeding rate | Seed cost
(2012 price) |
|---|---|---|---|---|---|---|---|---|
| Lentil | Fast growth; smothering effect on weeds; good biomass production; nitrogen restitution in spring | Seeds need to be well buried and soil firmed | -3°C | Spreading, effective soil cover | Low | Host species | 25 to 30 Kg/ha | 1.95€/kg
i.e. 50 to 60 €/ha |
| Spring faba bean | Very efficient rooting: taproot that improves soil structure; nitrogen restitution | Sensitive to dry conditions, difficult emergence; cover seeds 4 to 5 cm; large seeds, mixture becomes delicate | -2°C | Upright, low soil cover | Low | Host species | 100 to 115 kg/ha | 0.80 €/kg
i.e. 80 to 95€/ha |
| Nyjer | Very frost sensitive plants; soil restructuring | Delicate establishment: shallow sowing (1 cm), fine and well-firmed seedbed; low weed competitiveness; high heat requirement | -1°C | Upright, incomplete soil cover | Medium to good | Host species | 20kg/ha | 3.70 €/kg
i.e. 75 €/ha |
| Camelina | Allelopathic effect on weeds | Same family as rapeseed; competitive with rapeseed if sown at high density; difficult frost destruction: requires chemical herbicide | -10°C | Upright but smothering plant | Host species + risk of cabbage root fly | 1 Kg/ha to use in mixture | 11.90 €/Kg
i.e. 35€/ha | |
| Buckwheat | Easy frost destruction from first frosts; drought tolerant; deep rooting: good soil restructuring; strong competition with weeds: smothering and allelopathic effect | Rapid flowering; no symbiotic nitrogen fixation | 0°C | Upright | No | No | 30 to 40 kg/ha | 1.90€/Kg
i.e. 57 to 76 €/ha |
| Spring pea | Good autumn growth: strong weed competitiveness; good soil restructuring; significant nitrogen restitution (60% of trapped nitrogen) | Aggressive plant (may hinder rapeseed if vigorous growth); need to cover seeds; expensive seeds; seed size (large seeds making mixture difficult for homogeneous sowing) | -2°C | Upright but more or less covering plant | Low | Host species | 50 to 70 Kg/ha | 1.30€/kg
i.e. 65 to 90 €/ha |
| Alexandrian clover | Annual plant of this family; easy to destroy; rapid development; soil restructuring thanks to fasciculate-taproot root system | Medium weed competitiveness; drought sensitivity | -1°C | Upright | Good | Host species | 15 kg/ha | 2.60 €/Kg
i.e. 40 €/ha |
| Common vetch | Good drought resistance; soil restructuring by taproot; good soil cover and rapid development | Heterogeneous development: need to associate this species (lentil, faba bean or fenugreek); expensive seeds | -7°C | Spreading | Medium | Host species | 25 kg/ha in association | 1.90 €/Kg i.e. 50 €/ha (vetch only) |
| Fenugreek | Rapid development; low water requirement (heat and drought tolerant); non-invasive; nitrogen restitution | Attracts game due to its odor | -7°C | Upright | ./ | 10 Kg/ha in association | 1.70 €/kg
i.e. 17 €/ha (fenugreek only) | |
| Common spring vetch | Good soil cover if associated with purple vetch; possible nitrogen restitution | Need destruction for non-frost hardy varieties (threshold = -10°C) | -2°C | Spreading | Low | Host species | 20 Kg/ha | 1.40 €/kg
i.e. 28 €/ha |
| Annual medic (alfalfa) | Good soil cover; possible nitrogen restitution | Must be surface sown to germinate; drought sensitivity | -3°C | Spreading | Low | ./ | 3 to 5 Kg/ha | 6.70 €/kg
i.e. 20 to 35 €/ha |
Lentil
- Advantages:
- Fast growth.
- Limited height.
- Shallow rooting.
- Good competitiveness against weeds.
- Nitrogen mineralization in spring.
- Disadvantages:
- Frost sensitivity: -7°C.
- Susceptible to Aphanomyces.
- Host species for sclerotinia.
- Poor selectivity to chemical weed control.
- Seeding rate in association / Objective plants/m²: 25 to 30 kg/ha. Target 100 to 130 plants/m².
- Indicative cost: 50 to 60 €/ha.
Spring faba bean
- Advantages:
- Rapid development.
- Very efficient rooting, structures soil, improves water circulation.
- Biomass production.
- Nitrogen restitution.
- Use farm-saved seeds.
- Frost sensitivity.
- Good selectivity to chemical weed control.
- Disrupts insects in autumn.
- Disadvantages:
- Sensitive to dry conditions.
- Large seeds: delicate mixture with rapeseed.
- Seed cost.
- Host species for sclerotinia.
- Poor soil cover.
- Seeding rate in association / Objective plants/m²: 80 to 100 kg/ha. Target 10 to 15 plants/m².
- Indicative cost: 20 to 25 €/ha (farm-saved seeds).
Alexandrian clover
- Advantages:
- Annual plant.
- Easy to destroy.
- Competitive against weeds.
- Nitrogen mineralization in spring.
- Soil restructuring thanks to its fasciculate-taproot root system.
- Good selectivity to chemical weed control.
- Disadvantages:
- Medium ground cover.
- Drought sensitivity.
- Host species for sclerotinia.
- Seeding rate in association / Objective plants/m²: 6 kg/ha. Target 200 to 300 plants/m².
- Indicative cost: 15 €/ha.
Dwarf white clover
- Advantages:
- Rapid development.
- Good soil cover.
- Good selectivity to chemical weed control.
- Disadvantages:
- Little frost sensitivity.
- Seeding rate (kg/ha) if species alone: 4-5.
- Cost €/ha: 30 €/ha.
White clover develops very little in autumn and thus provides little benefit to rapeseed. It is frost-tolerant, with growth starting in spring. This specificity means it can be added in a mixture of frost-sensitive legumes to develop rapidly during the following intercrop period, or even serve as a permanent cover remaining in place in subsequent crops. To limit competition risks with rapeseed in spring, it is imperative to choose dwarf varieties such as Aberace. In systems with semi-permanent cover, it is recommended to sow the cover simultaneously with rapeseed rather than sow rapeseed into an existing cover (increased risk of water competition). Care should be taken with the sowing depth of white clover, which must be positioned superficially.
Spring pea
- Advantages:
- Rapid development.
- Good soil cover.
- Frost sensitive.
- Disadvantages:
- Seed cost.
- Medium selectivity to chemical weed control.
- Seeding rate (kg/ha) if species alone: 50-80.
- Indicative cost: 15 €/ha (farm-saved seeds).
Spring flax
- Advantages:
- Frost sensitive.
- Good selectivity to chemical weed control.
- Seeding rate (kg/ha) if species alone: 1.
- Indicative cost: 1.3 €/ha.
Triple or more mixtures
- Advantages:
- Good complementarity between species.
- For some mixtures, seeds of similar size can be mixed directly with rapeseed.
- Disadvantages:
- Some species may dominate others.
- Beware of difficulty in frost destruction for species like vetches or bitter vetch.
- Seed cost.
- Indicative cost: 40-80 €/ha.
Associating several legumes is interesting to multiply advantages and offset the drawbacks of one species. Upright, later species with taproots (faba bean) can be associated with more spreading plants with fasciculate roots and often earlier (lentil, clover Alexandrian, etc.).
Examples of mixtures frequently associated with rapeseed:
- Faba bean + Vetch + Alexandrian clover
- Bitter vetch + Fenugreek + Lentil (recommended mixture in low frost risk areas)
- Faba bean + Lentil
- Bitter vetch + Lentil + Alexandrian clover (recommended mixture in low frost risk areas)
Warning: Vetch is not frost sensitive enough, bitter vetch is poorly covering and adds nothing more, and fenugreek is poorly covering and adds nothing more except as a repellent against hares.
To remember:
- To fight against voles: 2kg of melilot mixed with the cover.
- Repellent against hares: 10 to 20kg of fenugreek mixed with the cover.
Association with perennial covers
Interest in permanent cover crops is growing. While it is risky to establish rapeseed in an existing cover (soil dried out by the cover), simultaneous establishment with rapeseed is relevant.
Benefits
Perennial legumes (lotus, white clover, red clover, alfalfa, etc.) develop little or not at all in autumn. They will not provide service to rapeseed. Their interest will be to cover the soil and produce biomass from rapeseed harvest and possibly provide various services to the following crop(s).
Points of attention
- Cover choice: lotus and white clover are species with the least risk of competition with rapeseed in spring.
- Delicate weed control management: perennial legumes do not help suppress weeds while herbicide dose reduction is necessary and early winter catch-up treatments should be avoided to prevent their destruction.
It is therefore recommended to:
- Avoid fields with high dicotyledonous weed pressure.
- Associate frost-sensitive legumes with good early soil cover capacity: single-cut Alexandrian clover or lentil.
Establishment
The size difference between rapeseed seeds and those of the cover crop can be difficult to manage during sowing. Several methods exist depending on the nature of the associated cover crop and the equipment.
| Sowing type | Seed type | Possible species |
|---|---|---|
| Sowing with the same
seeder with 2 hoppers |
Any type | All
species |
| Broadcast sowing of the associated cover
(then incorporation with seeder or tool) |
(spreading width too narrow)
|
Lentil,
field bean |
| Sowing with the same
seeder in mixture with rapeseed |
Small seeds | Lentil,
clover, |
| Row sowing
with microgranulator |
Density <30kg/ha | Lentil,
clover, |
Warning: If the associated crops are sown too early, there is a risk of excessive cover crop growth which may smother the rapeseed.
Establishment advice / Crop management adaptation
- Perform early sowings, between August 15 and 20 (5 days before the normal sowing date for rapeseed alone), to promote the growth of companion plants and increase their sensitivity to frost when the first cold spells arrive. Conversely, late sowing will only allow limited development of companion plants and will not justify the additional cost of seeds.
- Choose a variety with low sensitivity to elongation.
- Sow simultaneously and mixed with the rapeseed seed for seeds of similar density (e.g., rapeseed + lentil). This saves a pass if the equipment allows.
- Establish in two passes for seeds with different density from rapeseed, sow the cover crop before the rapeseed. Do not wait too long between the two sowings to avoid destroying the cover crop and prevent the association from overtaking the rapeseed. For time reasons, broadcast sowing will be the most advantageous.
- Choose a field free of dicotyledonous weeds, especially geranium. The herbicide strategy should be adapted and limited to one early pass or two to reduce phytotoxicity on companion plants (see below "Adapt the herbicide program").
- Nitrogen fertilization in spring can be reduced, generally by 30 units. All these management adaptations ensure the success of the practice and justify the additional cost of companion plant seeds.
Weed management
The phytotoxicity of rapeseed herbicides can significantly harm the associated cover crop, even destroying it. Care must be taken regarding the products and treatment doses applied.
Reducing herbicides combined with the presence of an associated cover crop can help achieve clean fields with less intensive weed control programs.
Grass weed control
- Post-emergence catch-up: same reasoning as for rapeseed alone, on grass regrowth with a foliar grass herbicide.
- Catch-up at winter entry or exit: with a root herbicide.
Broadleaf weed control
- Doses and application stages differ from rapeseed alone. Pre-sowing applications are discouraged and pre-emergence applications should be avoided if possible as they are less selective than post-emergence applications.
- Favor applications at the rosette stage or 2 to 4 leaves of rapeseed. Split into 2 passes if necessary.
The main causes of failure of the associated rapeseed technique are related to weed control: these are situations with a history of high weed pressure or herbicide failure. Failures can also result from competition on rapeseed by certain associated cover crops of vetches or clovers (purple, crimson, Alexandrian) not destroyed at winter exit.
Here is a summary table on this subject, produced by the chambre d'agriculture of Burgundy
Disease management
Care must be taken not to increase the soil infectious potential regarding fungal issues (aphanomyces) in the presence of legumes in the rotation. Some legumes increase this risk (lentils, vetch, pea, clovers, vetches), others less so (field beans, fenugreek, Alexandrian clover).
See also
- Crop associations and rapeseed in Hauts-de-France
- Associating rapeseed with frost-sensitive service plants
- Successful associated rapeseed in conservation agriculture, with Antonio Pereira
- ASSOCIATED RAPESEED With field bean, easy organization and proven results @CA17TV
Further reading
Videos
Sources
- Associating a cover crop with rapeseed - Chamber of Agriculture of Meuse
- Successful establishment of associated rapeseed - Chamber of Agriculture of Meuse
- Benefits and management of rapeseed associated with legumes - Terres Inovia
- Associated rapeseed: a tool to choose companion plants - Entraid'
- Associated rapeseed: How? What advantages? - Terre-net
- Associated rapeseed: diversity serving performance, AgroLeague
Cette technique s'applique aux cultures suivantes
La technique limite la présence des auxiliaires et bioagresseurs suivants
- ↑ Growing winter rapeseed in association with service plants - CA Poitou-Charentes - 2021 https://agriculture-de-conservation.com/sites/agriculture-de-conservation.com/IMG/pdf/colza-associe-poitou.pdf
